The utilization of the amino acid L-histidine by streptomycetes is controlled at the levels of gene expression and enzymatic activity. Histidine ammonia-lyase (histidase), the enzyme that catalyzes the first step in the conversion of histidine to glutamate, is regulated post- translationally in Streptomyces griseus by the concerted action of at least three proteins. One of these proteins can be replaced in vitro by a phosphatase, and another can be replaced by a phosphodiesterase. The proposed experiments have been designed to identify and characterize the genes and gene products involved in the control of histidase activity in S. griseus. The analysis of mutants that cannot activate histidase will facilitate the elucidation of the mechanism by which histidase is activated in vivo. The inactive and active forms of histidase will be identified by a combination of electrophoretic and immunological techniques. To determine whether histidase activity is regulated in response to changes in the streptomycete life cycle, fluctuatuations in the relative amounts of these forms during differentiation will be measured. Activation factor, the protein that converts histidase to the catalytically active form, will be purified, its activity characterized, and its structural gene cloned. The histidase structural gene will also be cloned, to facilitate analysis of the regulation of histidase activity and to permit the identification of molecular features of histidase gene expression. By these studies, we will gain a more detailed understanding of molecules involved in the regulation of nitrogen metabolism in streptomycetes. This research may also have ramifications extending to a regulatory processes functional during cellular differentiation and secondary metabolism.